1. Field of the Invention
The present invention relates to earth boring drill bits. In particular, the invention relates to an improved geometry for cutting element compacts that are installed in corresponding holes of rolling cone earth boring drill bits.
2. Description of the Related Art
A drill bit fitted with one, two, or three rolling cones may be employed for drilling a borehole by the rotary method in an earthen formation. The drill bit is secured to the lower end of a drill string that is rotated either from the surface or by downhole motors or turbines. The rolling cones are mounted on the drill bit and have cutting elements that contact the interior of the borehole as the drill string is rotated, thereby engaging and disintegrating the earthen formation material. One type of cutting element comprises compacts that are partially disposed within holes in the cones and used to penetrate, gouge and scrape the bottom and sidewalls of the borehole. The cuttings from the borehole are carried in suspension and washed to the surface by drilling fluid that is pumped down from the surface through the hollow, rotating drill string.
When drilling hard and abrasive formations, the life of a drill bit is frequently limited by, among other factors, the wear rate of the compacts in the rolling cones. It is not unusual for the compacts to become worn and lose their ability to effectively cut and penetrate through formations. It is also not unusual for the wear on the compacts to be so severe that the compacts become dislodged after long periods of drilling. In general, worn compacts result in a shorter bit life, and a shorter bit life translates directly into higher well drilling costs, as the entire drill string must be removed from the borehole in order to replace the bit. This process is expensive and requires a substantial amount of time and effort. Thus, it is desired to have a drill bit that can be utilized for longer periods of time and does not need to be replaced as frequently.
It has been previously proposed to extend the life of drill bits by increasing the size of the exposed cutting area of the compact that contacts the formation materials. However, this has typically also involved increasing the size of the entire unexposed area of the compact disposed within the cone cutter surface. An increase in the size of this entire unexposed area results in a decrease in the amount of supporting metal available between the compacts. A minimum amount of supporting metal is required to surround the unexposed portion of each compact to retain the compact within the cone cutter surface.
It has also been previously proposed to reduce the wear on particular compacts by using more compacts in the rolling cones. However, if the rolling cones remain the same size, then adding additional compacts would require that the compacts be spaced more closely together. This has been shown to slow the rate of drill bit penetration and does not fundamentally change the wear characteristics at the compacts. Further, it does not leave adequate supporting metal available between the compacts, and on occasion allows the unexposed portions of the compacts to intermesh or overlap with one another, thus reducing compact retention capability.
A need exists, therefore, for an earth-boring drill bit with compacts and corresponding holes that have an improved geometry that will result in a reduced bit wear rate and increased bit retention during drilling operations.